Axial flow steam turbines generally include fixed stator blades, rotating rotor blades, between-blade seal fins, and a casing surrounding them. The seal fins are commonly retained in circumferential grooves in the casing by caulking wire.
The fins lose their sealing efficiency through wear and need to be refurbished to maintain turbine performance. This typically involves removing the turbine to a workshop, separating the casing into half-casings, and removing both the stator and rotor blades to access the seal fins and caulking wire. The old fins are then machined down to access the caulking wire which is carefully machined out to avoid damaging the grooves of the half-casings. Replacement fins and caulking wire are then mounted in the grooves, and the replacement fins are machined down to precise clearances from the rotor shaft and blades.
Precision machining of seal fins and caulking wire is extremely difficult because the half-casings deform when the casing split joint (or horizontal half-joint) is opened due to stress changes created after years of use. This causes the radius of curvature of the grooves, and hence the radial position of the seal fins and caulking wire, to differ at different points around the inner diameter of the half-casings. For example, the split half-casings can be up to 1.5 mm out of round.
It would be advantageous to refurbish caulked-in seal fins on-site where the turbine is normally operated, and with the stator blades in situ in the half-casings.
A need therefore exists for a boring machine for on-site refurbishment of seal fins caulked in turbine half-casings having a variable radius of curvature.